1. Field of the Invention
This invention relates to a lithographic projection apparatus and a method for manufacturing a device using a lithographic projection apparatus.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g. a silicon wafer). The lithographic apparatus comprises an illumination system to illuminate the mask and a projection system (also referred to as a projection lens) to transfer the pattern, via imaging, onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Known lithographic apparatus include so-called steppers or step-and-repeat apparatus, and so-called scanners or step-and-scan apparatus. In a stepper, each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and the wafer is moved by a predetermined amount to a next position for a next exposure. In a scanner, each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction, and next the wafer is moved to a next position for a next exposure.
With a scanner, a mask-area irradiated by the illumination system is typically a slit shaped rectangular or elliptical area, whereby in the non-scanning direction the size of the irradiated mask area corresponds to a maximum object field size of the projection lens. In such a scanning exposure apparatus, since the exposure region has a rectangular or slit-like shape, lens elements of the projection optical system may be irradiated in areas of a substantially rectangular or elliptical shape. At a lens element, the footprint of the radiation traversing the projection system is determined by the location where the lens element is disposed in the optical path. In particular, lens elements placed away from a pupil plane of the projection lens are subjected to irradiation in an asymmetric, elongated area.
Due to residual absorption of projection beam radiation in the optical elements and their surface coatings, lens heating is induced during one or more exposures and such a heating may cause rotationally asymmetric deformation of projection lens elements and produce a rotationally asymmetric optical aberration. For example, the optical aberration may include a beyond-tolerance amount of third and higher order astigmatism, whereby the position of best focus for a line-shaped feature of the pattern may become dependent on the orientation of the line. Astigmatism may severely degrade a device pattern image.
A technique for alleviating the induced aberration effect of asymmetric lens heating due to the slit shape of the irradiated mask area is to provide additional radiation beams of non-actinic wavelength to the projection lens so that the lens heating is more uniform. An alternative technique is to define a desired illumination mode (i.e., the effective intensity distribution in the illumination system pupil) by means of different, crossed linear polarizers, one polarizer being disposed in the illumination system and the other (crossed) polarizer being disposed in the projection system. Lens elements upstream of the polarizer in the projection system are then irradiated more uniformly compared to an arrangement where the illumination mode is defined entirely in a pupil of the illumination system.